Ellipsometry is a measurement technique allowing the study of properties of reflecting bodies from the knowledge of the reflection effects on the state of polarization of a light beam. Ellipsometry exploits the fact that, upon directing polarized light onto a surface there is a variation in the relative phase between a component polarized in the incidence plane and the component polarized in the perpendicular plane, and a variation in the ratio between the amplitudes of the two components. By indicating by .beta..sub.pi, .beta..sub.si, .beta..sub.pr, .beta..sub.sr the phases of the components of the incident and the reflected beam polarized in the incidence plane and in the perpendicular plane, respectively, and by Ep, Es, Rp, Rs the amplitudes of the components of the incident and reflected beam, an ellipsometer allows measurement of angles: ##EQU1##
The values of .DELTA., .psi. depend according to known relations on the properties of the body under test, more particularly on the thickness and the refractive index. Considering a body mounted on a substrate and assuming that Es=Ep, the following relation applies: ##EQU2## where r.sub.01 is the reflection coefficient at the separation surface between the medium (e.g. air) in which the test body is immersed and the body itself; r.sub.12 is the reflection coefficient at the separation surface between the body and the substrate; subscripts p,s indicate that the reflection coefficient is relevant to the wave polarized in the incidence plane or in the perpendicular plane; .delta.=(360/.lambda.).multidot.d(n.sub.1.sup.2 -sen.sup.2 .phi.).sup.1/2 is the phase shift undergone by rays reflected by the substrate at each crossing of the body with thickness d and refractive index n.sub.1 with respect to the rays reflected by the surface of the body, .phi. being the incidence angle of the beam on the body.
Relationship (1) takes into account multiple reflections on the separation surface between the body and air and between the substrate and the body.
Reflection coefficients r.sub.01, r.sub.12 are tied to the refractive indices of the different media by the well-known Fresnel formulae which are given here for r.sub.01 : ##EQU3## where .phi..sub.1 is the refraction angle of rays with incidence angle .phi..
Many commercially available ellipsometers are based on the extinction principle: the light emitted by a source and polarized in a polarizer traverses a compensator which is adjusted so as to introduce equal and opposite phase-shifts to those caused by reflection, thereby originating a linearly-polarized radiation; the latter, after reflection, is collected by an analyser which is in turn adjusted so as to cause the extinction.
The magnitude of the compensator adjustment gives .DELTA. and that of the analyzer adjustment gives .psi.. It is clear that the measurement by these devices is very slow, since the compensator and analyzer adjustments interact with each other.
These disadvantages are overcome by ellipsometers based on interferometric techniques. An ellipsometer of this kind is described in "Ellipsometry and polarized light" by R. M. Azzam and N. M. Bashara, North-Holland Publishing Company, 1977, pages 262 to 265, and in the article "Automated laser interferometric ellipsometry and precision reflectometry" by H. F. Hazebroek e W. M. Wisser, Journal of Physics, Section E, Vol. 16, 1983.
In such an ellipsometer a polarized beam of radiation is divided by a beam-splitter into two beams. One beam is sent towards the sample under test and is reflected on a mirror by which it is retroreflected towards the sample and the splitter; the other, acting as reference beam, is sent to a corner reflector, reflected onto a mirror and sent back from here to the reflector and the splitter. The two beams are recombined by the splitter into a single beam whose components parallel and perpendicular to the plane of incidence on the sample are separated and sent to different detectors. The corner reflector is translated at a constant speed so as to cause a frequency variation in the reference beam by Doppler effect.
This interferometric ellipsometer has a number of disadvantages. More particularly, the corner reflector position is critical, as it has to be chosen so as to make incident reference radiation coincide with one of the two reflector self-polarizations, to maintain the reference radiation polarization. The position of the mirror in the measurement branch is critical too, since the mirror must be accurately perpendicular to the light reflected by the sample to prevent polarization variations and to make the two reflections take place on the sample at the same point. Besides, the ellipsometer includes moving parts which always entail reliability problems.